1. Field of the Invention
The present invention relates generally to measuring the consumable electrode length in an electric arc melting furnace. More particularly, it relates to measurement of the tip location of a consumable electrode using fiber optic technology and providing high temperature capability to an optical fiber.
2. Description of the Related Art
In the steel industry, measurement of the length of the consumable electrode in electric arc furnaces is necessary to ensure minimum energy, material losses, and prevent shortout in the hot metal. In a submersible arc application, the tip of a consumable electrode is immersed in a pile of ore at some predetermined point. There is an arc produced which forms molten metal that flows out of the furnace through a slanted bottom or some regulated draining system that is well known in the art. It is important to know the exact tip location of the consumable electrode for optimal production.
To obtain the best results when using an arc operating device such as an arc furnace, or an arc welder it is imperative that the distance between the elements of the machine between which the arc forms is adjusted and controlled.
In the application at hand, it is compulsory to maintain the length of the arc at its optimal value by controlling the distance between the electrode and the molten metal especially when the electrode is continuously moving due to erosion of its tip.
Some methods require a manual "dipstick" or "sounding" measurement which are inefficient and non-continuous. However, process modeling requires a continuous input of the location of the electrode tip for process optimization. This position information is used not only to readjust tip location but to control, for example, the feed rate of fresh minerals and drain rate of molten metal. Inputs which are provided to the process model based on the infrequent manual soundings, and a prior knowledge of electrode consumption rates, are known to have inaccuracies and cumulative errors.
There have been many attempts in the prior art to find a way to accurately and precisely control and measure the position of the consumable electrode as it is being consumed in the furnace. Most of these attempts focus on the use of arc voltage or some electrical phenomenon generated from the arc voltage such as "hash" and "drop short" phenomena. One such reference, U.S. Pat. No. 4,303,797 employs a mathematical model based on electrode drive speed and voltage discontinuities to control the gap between the bottom of an electrode and the top surface of an ingot. A similar approach was used by Kjolseth, et al (U.S. Pat. No. 3,375,318) by employing the derivative of change in resistance with respect to the electrode position as determined for the location of the electrode point. Both of these references teach the use of some electric variable for setting up a mathematical model for controlling the position of a consumable electrode. Likewise, U.S. Pat. No. 3,187,078 teaches of using voltage discontinuities in a servo system for controlling the arc gap and U.S. Pat. No. 4,578,795 discloses a process for monitoring the gap voltage between the consumable electrode and the molten metal by monitoring the occurrence of drop shorts. The problem with these approaches is the difficulty of precise control due to the electrical noise. The problems encountered using an electrical approach are discussed in the previously cited references.
Even earlier approaches to monitoring the length of a consumable electrode included a signaling device consisting of a wire extending from a reel to the electrode. When the electrode is consumed up to the wire, the connection is broken causing the lamp, to which the wire is also connected, to go out. This signals the operator that the melting has progressed to the predetermined point. A slightly different approach in U.S. Pat. No. 3,379,818 used the weight of the remaining electrode as a signaling device.
A more recent attempt for controlling the length of an electrical arc in an arc generating machine employed acoustical signals in U.S. Pat. No. 4,435,631. This reference teaches that the acoustical signal generated by an arc is a function of the length of its column which can be compared to a reference signal. It also teaches to modulate the arc column supplying current, for a DC current, to generate an acoustical signal. This reference points out several drawbacks with the prior art for measuring arc voltage drop via an electrical voltage probe. The first drawback is that the electrical voltage probe is never electrically insulated from the arc power supply. This poses noise and drift problems when the arc generating machine is operating at high voltage or when the arc supplying current is floating. A second drawback is that the measuring loop is subject to parasitic voltages induced in the loop when the arc current undergoes large variation.
This reference also teaches of the difficulty of obtaining an exact value of the arc voltage drop. It teaches that it is almost impossible to isolate the arc column voltage drop from the value obtained with the measuring loop.
The disadvantages of employing an electrical approach for control extend to an acoustic approach that uses magnetostrictive wire. Since electrically conductive wire must extend down to the electric arc, there is still the presence of electrical noise. Likewise, the acoustic time-of-flight depends on wire temperature, which is unlike the present invention. Finally, distance, resolution and precision depend on pulse width. Optical pulses can be produced with picosecond width, while acoustic pulses cannot. To use acoustical signals in both AC and DC applications, additional equipment is required.
There is a need for an apparatus and method for measuring the length of a consumable electrode which has electrical noise immunity. There is also a need for an apparatus that provides an electrically noise-free link to the electrically hostile arc furnace.
Optical fiber length measurement using an optical time domain reflectometer (OTDR) is a method used primarily for fault or break location such as taught in U.S. Pat. No. 4,289,398, which is hereby incorporated by reference. It has not been suggested before to apply this technology to a consumable electrode tip measurement.